The answer to 'why is my furnace not working' is that there could be several common causes, including a tripped circuit breaker, a clogged air filter, or a problem with the thermostat. The furnace may not be receiving power, the air flow may be restricted, or the thermostat may not be properly controlling the furnace.
If your furnace is not working, first check the circuit breaker to make sure it has not tripped. Then check the air filter and replace it if it is dirty. You can also try adjusting the thermostat to see if that resolves the issue. However, if these basic troubleshooting steps don't fix the problem, it's best to call in a professional HVAC technician to diagnose and repair the issue.